So I have a 3.15gpa and will probably/hopefully be attending a lower-t14.

I plan to go for clerkship/biglaw ---> govt/non-profit/policy for several years after LS.

Say I decided to go get a masters in pub policy, history, polisci, etc 10 years after law school (maybe full time, maybe part time.)

Regardless of whether or not you think this would be an intelligent decision (probably not) would my law school class rank/achievements be the major focus of graduate school admissions? Or is this 3.15 still going to be a deterrent for top programs?

In history/polisci (and probably most related programs), it's a lot more holistic than law school (in part because though USNWR technically ranks these programs, no one cares - it's about who you work with/what work you do, rather than just the school you attend). So they would certainly look at your UG GPA, but they'd consider your law school record, too, quite strongly. What they'd look for most is evidence that you have potential to do excellent research, so they would want to see some kind of track record of sustained study (and, ideally, some kind of scholarship - thesis, articles published, in this case probably law school seminar papers/notes). LORs are also more important than in law school, if the people writing can speak really specifically to your potential to succeed in the program (and know what they're talking about). If your graduate program ends up being in an area related to the government/non-profit/policy work you (hope to) do after law school, your success in that area will be more significant. UGPA is still a factor, but I would say in the scenario you describe, it would be much less so than for law school.

That said, I have to make the obligatory comment you didn't want made: why the hell would you get a master's in history or polisci? (I don't know enough to comment on the MPP.)

Personal satisfaction: perfectly reasonable reason (if you can afford it). Credential whoring: well, I suppose it never hurts to be able to say something like, "I have an MA from Yale," but I think it would be a really marginal benefit. (Because master's degrees in liberal arts subjects are basically useless for anything other than personal satisfaction. Again, don't know about MPP.) Also, keep in mind that sometimes the school that's really great for studying a particular subject may not actually have that much "lay prestige." But I realize you weren't really asking about that part, so I'll shut up now.

Reasoning at the moment is probably some combination of wishing I had studied those subjects in ugrad, personal knowledge, credential whoring, etc.

Whats wrong with studying those subjects by say reading a book? Or looking into the thousand other methods of learning a subject without schilling out thousands of dollars for a diploma that has zero, and I mean zero, positive impact on hiring in the legal or business world.